1. Field of the Invention
The present invention relates to a fuel pump for supplying fuel to an internal combustion engine or the like. This pump is used, for example, to supply fuel under pressure to a fuel injection system in an automobile or the like.
2. Description of Related Art
An automobile or the like having an engine equipped with an electronic fuel injection system employs a motor-operated fuel injection pump as one part of a device for injecting fuel into the engine. The fuel pump is submerged in a liquid fuel contained in a fuel tank and is designed to deliver fuel under high pressure to an injector in accordance with a command from an electronic controller.
One known type of such a fuel pump is generally called a regenerative pump or a Westco type pump. The efficiency, for example, of a Westco type fuel pump is highly dependent on the cross section of a flow passage section and the configuration of vanes of an impeller.
Japanese Patent Publication No. 63-63756, Japanese Utility Model Publication No. 3-2720, and Japanese Patent Laid-Open No. 60-47894, for example, disclose Westco type fuel pumps in which a desired level of performance is achieved by setting dimensions, such as the flow passage representative size Rm, to particular values.
A conventional Westco type fuel pump will be described below with reference to FIGS. 21 and 22. An impeller 9 of the conventional Westco type fuel pump has a disk-like outer configuration. A plurality of vanes 93 and a plurality of vane grooves 92 are provided alternately at equal intervals along the edge contacting one broad surface of the disk and its outer peripheral surface and also the edge contacting the other broad surface of the disk and the outer peripheral surface. These vanes and vane grooves are positioned on both sides of the impeller 9 with a partition wall 91 therebetween. An outer peripheral surface 910 of the partition wall 91 has a diameter equal to that of an outer peripheral surface 930 of the vane 93. A pump flow passage 95 is defined between an outer periphery of the impeller 9 and an inner surface of a pump casing 90. When the impeller 9 rotates, the outer periphery of the impeller 9 passes through the pump flow passage 95 at a high speed. Therefore, centrifugal forces on liquid in the vane grooves 92 form two vortexes 941, 942 in the pump flow passage 95. With the rotation of the impeller 9, the liquid fuel in the pump flow passage 95 is delivered in a circumferential direction while forming the two vortexes 941, 942 and is pressurized.
With the conventional Westco type fuel pump, however, a dead zone 96 is produced between the two vortexes 941, 942 as shown in FIG. 22. In the dead zone 96, the liquid fuel does not have a sufficient flow speed, thereby causing a counter flow. This raises the problem that the counter flow prevents fuel from being delivered under high pressure.
To eliminate the counter flow, it could be contemplated to provide a projection radially extending from the casing side or the impeller side in such a manner as to fill the dead zone 96. However, providing such a projection to fill the dead zone 96 gives rise to a fear that the pressure of the fuel might be uneven between opposite lateral sides of the impeller, because the fuel cannot move between the opposite lateral sides.
Westco type pumps are also used for applications other than fuel pumps. Japanese Patent Laid-Open No. 61-210288, for example, discloses a Westco type water pump. The technique disclosed in this document is intended to suppress the counter flow produced in the pump flow passage due to the presence of the dead zone. This document proposes that the distal end of the impeller's partition wall should be pointed. The disclosed prior art also proposes that the height of the impeller's partition wall should be lower than that of its vanes to position the distal end of the partition wall inside the vanes.
A Westco type pump with the height of the impeller's partition wall lower than that of its vanes to position the distal end of the partition wall inside the vanes, is also disclosed in Japanese Patent Laid-Open No. 56-32095 relating to an air pump.
However, the water pump disclosed in Japanese Patent Laid-Open No. 61-210288 or the air pump disclosed in Japanese Patent Laid-Open No. 56-32095 is greatly different from a fuel pump with regard to delivery capacity under pressure, impeller diameter and other factors. Therefore, if the disclosed techniques relating to water or air pumps are directly applied to fuel pumps, it would be difficult to achieve desired pump performance and operating effect.
A typical water pump, for example, requires a flow rate of 100 to 10,000 l/h and a pressure of 5 to 10 kgf/cm.sup.2. On the contrary, a typical fuel pump for automobile requires a flow rate of 50 to 200 l/h and a pressure of 2 to 5 kgf/cm.sup.2. Thus, parameter ranges required for practical operation of the pumps are different from each other to a large extent. Further, an impeller of a water pump is typically about 100 mm in diameter, while an impeller conventionally used in a fuel pump for automobiles is about 50 mm or 30 mm in diameter. The impeller size for a fuel pump is limited by its location in an automobile fuel tank.
In addition, an air pump is greatly different from a fuel pump not only in rated values of capacity, efficiency, impeller diameter, etc., but also in such characteristics as compressibility and viscosity of a target substance since a fluid to be pressurized by the air pump is gas. Thus, the air pump disclosed in Japanese Patent Laid-Open No. 56-32095 has a short radial distance between the vane distal ends of the impeller and the wall surface of the flow passage.
Furthermore, because the impeller diameter is large in water and air pumps, impellers are generally manufactured using metal materials. The metal impeller can be machined to cut the vane grooves for making the distal end of the partition wall pointed. On the contrary, because of its small diameter, the impeller of a fuel pump is generally molded by, for example, injection molding, using resin materials. This means that it is difficult to make the distal end of the partition wall pointed in the fuel pump, since deformations or cracks are often formed when a molding is released from molds. Particularly, a fuel pump having a smaller impeller diameter has the problem that a slight deformation of the impeller configuration affects the fuel flow passing through the flow passage and lowers pump efficiency. Consequently, there is a difficulty in achieving desired pump performance by directly applying the configuration of the conventional water or air pump to a fuel pump.